Page 371 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

326 Engineering Rock Mass Classification

Equation (26.11) is obtained by setting s1 ¼ s3 ¼ qtmass in Eq. (26.6). This represents a
condition of biaxial tension. Hoek (1983) showed that the UTS is equal to the biaxial
tensile strength for brittle materials.

Hoek (2007) proposed Eq. (13.5) for estimating rock mass strength (qcmass) from lab-
oratory strength of intact rock material (qc) and GSI for D ¼ 0.

MOHR-COULOMB STRENGTH PARAMETERS

Mohr-Coulomb’s strength criterion for a rock mass is expressed as

s1 À s3 ¼ qcmass þ As3 ð26:12Þ

where qcmass ¼ UCS of the rock mass, which ¼ 2 c cosf/(1 À sinf); c ¼ cohesion of the
rock mass; A ¼ 2 sinf/(1 À sinf); and f ¼ angle of internal friction of the rock mass.

Hoek and Brown (1997) made extensive calculations on the linear approximation of

non-linear strength criterion (Eq. 26.6). They found that strength parameters c and f de-
pend upon s3; thus, they plotted charts for average values of c (Figure 26.2) and f
(Figure 26.3) with D ¼ 0 for a quick assessment. It may be noted that c and f decrease

non-linearly with GSI unlike RMR (Table 6.10). The rock parameter mr may be guessed
from fp of a rock material at GSI of 90, if adequate triaxial tests are not done. Table 26.5
lists typical values of mr for various types of rock materials.

The angle of dilatancy of a rock mass after failure is recommended approximately as

D ¼ ðf=4Þ for GSI ¼ 75 ð26:13Þ
¼ ðf=8Þ for GSI ¼ 50
¼ 0 for GSI 30

FIGURE 26.2 Relationship between ratio of cohesive strength of rock mass to UCS of intact rock (c/qc)
and GSI for different mr values for D ¼ 0. (From Hoek and Brown, 1997)